1. Technical Field
The present invention relates to an oven controlled multistage crystal oscillator (hereunder, referred to as an “oven controlled crystal oscillator”), and in particular, to an oven controlled crystal oscillator with a high level of frequency stability.
2. Background Art
An oven controlled crystal oscillator is capable of maintaining a constant operating temperature of a crystal oscillator, and hence the level of frequency stability thereof is high. Therefore, it is applied to communication devices for base-station use in which frequency deviation of ppb (pulse per billion) units is required for example. In recent years, miniaturization is prevailing also in base-station use, and there has been available, for example, one that uses a chip resistor or the like as a heater element, instead of a conventional constant temperature heat cylinder with a heating coil wound thereon.
3. Prior Art
FIG. 6A, FIG. 6B, and FIG. 6C are drawings for describing an oven controlled crystal oscillator of a conventional example of the present invention, wherein FIG. 6A is a sectional view of the oven controlled crystal oscillator, FIG. 6B is an oscillation output circuit diagram thereof, and FIG. 6C is a temperature control circuit diagram thereof.
The conventional oven controlled crystal oscillator has a crystal vibrator 1, circuit elements 4 that constitute an oscillation output circuit 2 and a temperature control circuit 3, first and second circuit substrates 5a and 5b, and a metallic container 6. The crystal vibrator 1 has a crystal piece 1a of AT cut or SC cut for example, and it is, for example, seal-enclosed within a surface mount container 1b having external terminals.
In a case of either cut, the crystal piece 1a, with a limit value in the vicinity of 80° C. on the high temperature side at or above normal temperature 25° C., has a frequency-temperature characteristic in which oscillation frequency varies, depending the temperature. For example, AT cut gives a cubic curve shown with the curved line A in FIG. 7, and SC cut gives a cubic curve shown with the curved line B in FIG. 7. The vertical axis in FIG. 7 represents frequency deviation Δf/f where f denotes a frequency at a normal temperature and Δf denotes a frequency difference with respect to the frequency f.
The oscillation output circuit 2, as shown in FIG. 6B, includes an oscillating stage 2a serving as an oscillation circuit, and a buffering stage 2b having a buffering amplifier and the like. The oscillating stage 2a is of a Colpitts type having a voltage dividing capacitor and an oscillation transistor (not shown in the drawing) that constitute, together with the crystal vibrator 1, a resonance circuit. Here, for example, the oscillation output circuit 2 is of a voltage control type having a voltage-variable capacitive element 4a within an oscillating loop thereof. In FIG. 6B, reference symbol Vcc denotes power supply, reference symbol Vout denotes output, and reference symbol Vc denotes control voltage.
Furthermore, in the temperature control circuit 3, as shown in FIG. 6C, a temperature sensor voltage Vt from a temperature sensor element 4c (thermistor, for example) and a resistor 4d, is applied to one input terminal of an operational amplifier 4b, and a reference voltage Vr from resistors 4e and 4f is applied to the other input terminal. A voltage difference between the reference voltage Vr and the temperature sensor voltage Vt is applied to the base of a power transistor 4g, and electric power from the power supply Vcc is supplied to a chip resistor (heating resistor) 4h serving as a heater element. As a result, electric power to the heating resistor 4h is controlled with a resistor value that is dependent on the temperature of the temperature sensor element 4c, thereby maintaining the operating temperature of crystal vibrator 1 at a constant temperature.
The first and second circuit substrates 5a and 5b, as shown in FIG. 6A, both have a wiring pattern formed thereon, and for example, the first circuit substrate 5a is of a ceramic made substrate and the second circuit substrate 5b is of a glass epoxy made substrate. On one principal surface of the first circuit substrate 5a, there is arranged the crystal vibrator 1, and on the other principal surface, for example, there are arranged the heating resistor 4h and the temperature sensor element 4c of the temperature control circuit 3. On the heating resistor 4h and the temperature sensor element 4c, there is coated a thermally conductive liquid resin 7.
The second circuit substrate 5b is such that, as shown in FIG. 6A, on both principal surfaces thereof, there are arranged respective circuit elements 4, other than those described above, of the oscillation output circuit 2 and the temperature control circuit 3, and in particular, circuit elements 4 of the oscillating stage 2a are arranged in the central region thereof The first and second circuit substrates 5a and 5b are electrically and mechanically connected by metallic pins 8a, and there is provided a two-stage structure with the substrate surfaces facing each other. In this case, the thermally conductive resin 7 coated on the heating resistor 4h and the temperature sensor element 4c comes into close contact with the central region of the second circuit substrate 5b, and is thermally bonded to the second circuit substrate 5b, thereby maintaining the constant operating temperature of the circuit elements 4 serving as the oscillating stage 2a in particular.
The metallic container 6 shown in FIG. 6A is made airtight with glass 9 at least in four corner sections thereof, and it includes a metallic base 6a, through which lead wires 8b serving as so-called airtight terminals pass, and a metallic cover 6b sealed thereon by means of resistance welding or the like. The second circuit substrate 5b is electrically and mechanically connected to the lead wires 8b of the metallic base 6a, and is seal-enclosed within the metallic container 6 together with the first circuit substrate 5a. 
In such a conventional oven controlled crystal oscillator, the operating temperature of the crystal vibrator 1 that governs the frequency-temperature characteristic of the oscillation output Vout shown in FIG. 6B, is maintained constant, while maintaining the temperature of the respective circuit elements 4 that constitute the oscillation circuit (oscillating stage) 2a at a constant temperature. Consequently, the operating temperature not only of the crystal vibrator 1 but also of the oscillating stage 2a is maintained constant, and hence it is possible, without being influenced by their frequency-temperature characteristics, to obtain frequency stability with frequency deviation Δf/fo of at least ±0.01 ppm or greater, or of ppb units. Here, fo denotes a nominal value of oscillation frequency, and Δf denotes an amount of deviation from the nominal value fo. (Refer to Japanese Unexamined Patent Publication No. 2005-341191, and Japanese Unexamined Patent Publication No. 2005-223395)